The present invention relates generally to solvent extraction processes for recovery of metal values from aqueous solutions and, more particularly, to formulative procedures for developing improved solvent extraction reagents and to the use of such reagents in recovery of, e.g., copper values.
The starting material for large scale solvent extraction processing of copper is an aqueous leach solution obtained from a body of ore which contains a mixture of metals in addition to copper. The leaching medium dissolves salts of copper and other metals as it trickles through the ore, to provide an aqueous solution of the mixture of metal values. The metal values are usually leached with sulfuric acid medium, providing an acidic aqueous solution, but can also be leached by ammonia to provide a basic aqueous solution.
The aqueous solution is mixed in tanks with an extraction reagent which is dissolved in an organic solvent, e.g., a kerosene. The reagent includes an extractant chemical which selectively forms metal-extractant complex with the copper ions in preference to ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process.
The outlet of the mixer continously feeds to a large settling tank, where the organic solvent (organic phase), now containing the copper-extractant complex in solution, is separated from the depleted aqueous solution (aqueous phase). This part of the process is called phase separation. Usually, the process of extraction is repeated through two or more mixer/settler stages, in order to more completely extract the desired metal.
After extraction, the depleted aqueous feedstock (raffinate) is either discharged or recirculated to the ore body for further leaching. The loaded organic phase containing the dissolved copper-extractant complex is fed to another set of mixer tanks, where it is mixed with an aqueous strip solution of concentrated sulfuric acid. The highly acid strip solution breaks apart the copper-extractant complex and permits the purified and concentrated copper to pass to the strip aqueous phase. As in the extraction process described above, the mixture is fed to another settler tank for phase separation. This process of breaking the copper-extractant complex is called the stripping stage, and the stripping operation is repeated through two or more mixer-settler stages to more completely strip the copper from the organic phase.
From the stripping settler tank, the regenerated stripped organic phase is recycled to the extraction mixers to begin extraction again, and the strip aqueous phase is customarily fed to an electrowinning tankhouse, where the copper metal values are deposited on plates by a process of electrodeposition. After electrowinning the copper values from the aqueous solution, the solution, known as spent electrolyte, is returned to the stripping mixers to begin stripping again.
Many reagent formulations have been proposed for use in recovery of copper by solvent extraction in the prior art. See, e.g., Flett, D.S., "Solvent Extraction in Hydrometallurgy", Chemistry and Industry, No. 17, Sept. 3, 1977, pp. 706-712.
Reagents frequently employed in commercial processes for copper recovery are included among those offered by Henkel Corporation under the LIX.RTM. trademark, viz., LIX.RTM.63, LIX.RTM.65N, LIX.RTM.64, LIX.RTM.64N, LIX.RTM.70, LIX.RTM.71, LIX.RTM.73, LIX.RTM.34, LIX.RTM.54, LIX.RTM.605, LIX.RTM.617, LIX.RTM.622 and LIX.RTM.6022.
Briefly noted, LIX.RTM.63 includes, in addition to a liquid hydrocarbon diluent, an aliphatic .alpha.-hydroxy oxime extractant (5,8-diethyl-7-hydroxy-dodecan-6-oxime) of the type illustrated in Swanson U.S. Pat. No. 3,224,873. LIX.RTM.65N includes an alkyl substituted hydroxy benzophenone oxime (2-hydroxy-5-nonyl benzophenone oxime) as set out in Swanson U.S. Pat. No. 3,592,775, while LIX.RTM.64 and 64N incorporate benzophenone oxime extractants (2-hydroxy-5-dodecyl benzophenone oxime and 2-hydroxy-5-nonyl benzophenone oxime, respectively) in combination with an aliphatic .alpha.-hydroxy oxime as described in U.S Pat. No. 3,423,449. Formulation of LIX.RTM.70 involves combination of a benzophenone oxime extractant containing an electron withdrawing substituent (2-hydroxy-3-chloro-5-nonyl benzophenone oxime) with an aliphatic .alpha.-hydroxy oxime. LIX.RTM.71 and LIX.RTM.73 formulations both include a mixture of two benzophenone oximes, one of which has an electron withdrawing substituent (i.e., a mixture of 2-hydroxy-5-nonyl benzophenone oxime and 2-hydroxy-3-chloro-5-nonyl benzophenone oxime) with the latter reagent further including an aliphatic .alpha.-hydroxy oxime.
LIX.RTM.34 and LIX.RTM.54 incorporate alkaryl sulfonamido quinoline and .beta.-diketone extractants, respectively. LIX.RTM.605, LIX.RTM.617, LIX.RTM.622 and LIX.RTM.6022, on the other hand, employ alkyl substituted hydroxy benzaldoxime (salicylaldoxime) extractants according to Parrish, J. South African Chem. Inst., 23, pp. 129-135 (1970). Thus, LIX.RTM.605 and LIX.RTM.617 include 2-hydroxy-5-nonyl benzaldoxime extractants with, respectively, nonylphenol and tridecanol additives. LIX.RTM.622 and LIX.RTM.6022 comprise formulations of 2-hydroxy-5-dodecyl benzaldoxime and a tridecanol additive in approximately 4:1 and 1:1 w/w ratios, respectively. Acorga PT-5050 is offered for sale by Acorga, Ltd., Hamilton, Bermuda as a formulation comprising 2-hydroxy-5-nonyl benzaldoxime and a tridecanol additive in an approximately 2:1 w/w ratio. (See also, Ackerley, et al., U.S. Pat. No. 4,020,105; Ackerley, et al., U.S. Pat. No. 4,020,106; and Dalton, U.S. Pat. No. 4,142,952.)
Other reagents offered for commercial use in copper recovery by solvent extraction have included extractants of the hydroxy phenyl benzyl ketone oxime type (e.g., 2-hydroxy-5-nonyl phenyl benzyl ketoxime as illustrated in Anderson, U.S. Pat. No. 4,029,704, and offered for sale as P-17 by Acorga, Ltd.) and the hydroxy phenyl alkyl ketone oxime type (e.g., 2-hydroxy-5-nonyl phenyl methyl ketone oxime as described in U.K. Pat. No. 1,322,532 and offered for sale as SME 529 and SME 530 by Shell Chemical Company). SME 529 and SME 530 differ in that the latter contains an .alpha., .beta.-dioxime additive.
Also offered for commercial use in copper recovery are the Kelex 100 and Kelex 120 reagents of Ashland Chemical Company, which have included an hydroxy quinoline extractant (7-[3-(5,5,7,7-tetramethyl-1-octenyl)]-8-hydroxyquinoline as described in Buddie, et al., U.S. Pat. No. 3,637,711) either with a nonylphenol additive (Kelex 120 as described in Hartlage, U.S. Pat. No. 3,725,046) or without the phenolic additive (Kelex 100).
As may be noted from the above formulations, it is frequently the case that additive chemicals are included in commercial copper extraction reagents. Principal among the additives incorporated in the reagents are (1) kinetic additives and (2) modifiers of extraction and stripping equilibria. The kinetic additives (frequently referred to as "accelerators", "catalysts", "kinetic catalysts" or "kinetic synergists") may be defined as chemical substances included in solvent extraction reagents for the purpose of increasing the rate of transfer of metal values between organic and aqueous phases without materially affecting the position of equilibrium. Kinetic additives function to alter transfer rates in a variety of ways which are as yet far from completely understood. Reagent formulations such as LIX.RTM.64N incorporate a major proportion of benzophenone oxime and a minor proportion of an aliphatic .alpha.-hydroxy oxime with the result that the rate of extraction for the reagent under specified conditions may be significantly greater than that of the benzophenone oxime alone.
Modifiers of extraction and stripping equilibria are frequently incorporated in those commercial reagent formulations which include the so-called "strong" extractants. Such extractants are capable of forming a very stable complex association with copper at quite low pH's and, consequently, require the use of very highly acidic aqueous stripping solutions in order to effect the breakdown of the copper-extractant complex. Where extreme acidity of stripping solutions generates problems in employing conventional electro-deposition processes, modifiers are incorporated to shift equilibria in a manner facilitating stripping at lower acidities and to enhance overall metal extraction efficiency. A wide variety of modifier chemicals has been proposed for use in formulation of solvent extraction reagents for copper. These have included: long chain (C.sub.6 to C.sub.20) aliphatic alcohols such as isodecanol, 2-ethylhexanol, and tridecanol; long chain alkyl phenols such as nonylphenol (see, e.g., Hartlage, U.S. Pat. No. 3,725,046); and various organophosphorous compounds such as tributylphosphate (see, e.g., Ritcey, et al., Transactions of the International Solvent Extraction conference, 1974, pp. 2437-2481). Equilibrium modifiers most frequently employed in commercial reagents include nonylphenol (e.g., Kelex 120, LIX.RTM.605) and tridecanol (e.g., LIX.RTM.617, LIX.RTM.622, LIX.RTM.6022, PT-5050).
The use of kinetic additives and equilibrium modifiers has not been without drawbacks in the overall efficiency of solvent extraction processes in terms of the long range stability of reagents and the sensitivity of reagents to contaminants in aqueous feedstocks. As an example, while the minor proportion of kinetic additive present with the hydroxy aryl ketoxime extractant in the LIX.RTM.64N reagent formulation provides for kinetic enhancement in the use of the ketoxime, the additive is less stable toward hydrolytic degradation than the ketoxime. When used under operating conditions which are optimal for ketoxime extractant efficiency, the aliphatic .alpha.-hydroxy oxime thus tends to be depleted from continuous systems more rapidly than the ketoxime. Similarly, hydroxy aryl aldoxime extractants are less stable in use than ketoximes and are rendered even more unstable by the presence of large quantities of nonylphenol or other alcoholic modifiers. Alkyl phenol equilibrium modifiers, when entrained in aqueous phases have been noted to have severe deleterious effects on structural components of solvent extraction facilities, such as rubber linings, fittings, valves and the like. Finally, leach solutions containing dissolved silica frequently tend to form emulsions with active metal extractants and their diluents and the sensitivity of reagents to silica has been noted to be enhanced by the presence of equilibrium modifiers. With regard to the last-mentioned factor, it has frequently been noted (if not experimentally quantified) that kinetic and equilibrium modifying additives appear to have adverse effects on solvent extraction systems in terms of what is commonly referred to as "crud formation". Briefly put, solvent extraction systems used to treat agitation leach solutions and colloidal silica-containing leach solutions frequently display a build-up of semi-solid materials at the organic-aqueous interface. This "crud", if allowed to build uncontrollably, will eventually interfere with the solvent extraction process by decreasing the effective settler area, or in some cases will begin to pass from the settler to the next mixer and in the process cause significant cross-contamination problems. The build-up of crud is normally controlled by mechanical means which may involve a shutdown of the plant and subsequent loss of production.
On the whole, therefore, reagents which include minimal quantities of kinetic additives and/or equilibrium modifiers (or none of these additives at all) are among the most preferred and sought-after for commercial use in recovery of copper.
Of interest to the background of the present invention are those prior patents and publications which disclose reagent formulations which are kinetic additive and/or equilibrium modifier free.
Ashbrook, et al., U.S. Pat. No. 3,455,680 states that superior copper extraction results are obtained through use of assertedly synergistic combinations of aliphatic .alpha.-hydroxy oxime extractants and organophosphoric acid extractants. More particularly, the mixture of the two "weak" extractants, which ordinarily form stable copper-extractant complexes at relatively high acid pH's, is said to provide for more efficient extraction at lower pH levels than can be obtained with either extractant alone with the apparent elimination of the need for equilibrium modification. German Offenlegungsschrift No. 2,407,200, states that mixtures of two or more hydroxyphenyl alkyl ketoximes (of the type disclosed in the previously-noted U.K. Pat. No. 1,322,532) are distinguished by greatly improved extraction kinetics over the individual components and the apparent elimination of the need for use of kinetic additives. U.K. No. 1,537,828 asserts that synergistic kinetic effects attend incorporation of hydrocarbyl hydroxymethyl ketone oximes in reagent formulations including 2-hydroxy phenyl alkyl ketone oximes and/or 2-hydroxy phenyl aldoximes and/or 2-hydroxy benzophenone oximes. See, also, Canadian Pat. No. 1,083,828.
By and large, the literature has been less encouraging in its proposals for any enhanced effects resulting from the admixture of extractants with consequent decrease in relative content or elimination of kinetic additives or equilibrium modifiers.
In published evaluations of the LIX.RTM.71 and LIX.RTM.73 reagents (which, as noted above, comprise equimolar mixtures of a "strong", chlorine-substituted hydroxy benzophenone oxime with a relatively "weaker" benzophenone oxime not having an electron-withdrawing substituent), no kinetic or equilibrium shifting efficiencies in solvent extraction processing were noted. Properties of the mixed-extractant reagent were noted to be simply intermediate to those of the individual components. See, Agers, et al., TMS Paper No. A72-87, The Metallurgical Society of AIME, New York, N.Y. (1972). In discussion of combinations of .beta.-diketone extractants for copper and other metals, Marcus, et al., noted that most combinations provide no marked synergistic effects and maintained that this is in keeping with a prior hypothesis of synergistic effects only in the combination of one strong and one weak .beta.-diketone extractant. [See, "Ion Exchange and Solvent Extraction of Metal Complexes", pp. 853-854, Wiley-Interscience, London (1969).] The hypothesis is apparently inconsistent with the synergistic results assertedly obtained by admixture of equal strength hydroxy phenyl alkyl ketoximes in German Offenlegungsschrift No. 2,407,200 (kinetic enhancement) or admixture of two weak extractants in Ashbrook, et al., U.S. Pat. No. 3,455,680 (equilibrium shifting).
An evaluation of mixtures of LIX.RTM.34 (containing a quinoline extractant and no kinetic additive or equilibrium modifier) with LIX.RTM.64N (containing an hydroxy aryl ketoxime with an aliphatic .alpha.-hydroxy oxime kinetic additive) was provided by Kordosky in [CIM Special Volume 21, pp. 486-495 (1979)] and the mixture of extractants was noted to be apparently weaker in extractive power than either of the components individually. In Dissertation Abstracts International, Vol. 41, No. 11, p. 4233, May 1981, Valdes reports that mixtures of LIX.RTM.64N and SME 529 (containing an hydroxy phenyl alkyl ketoxime and no kinetic additive) were completely compatible but that no synergistic effects were noted.
Reagent mixtures of ketoxime and aldoxime extractants have been evaluated in contexts independent of their kinetic and equilibrium properties. As an example, Hanson, et al. [AIME Pre-Print No. 80-93 (Feb. 24-28, 1980)] reported on the comparative hydrolytic stability of, inter alia, mixtures of commercial reagents including LIX.RTM.63, LIX.RTM.65N, SME 529 and P5100. The results and conclusions of this work were expanded upon by Whewall, et al. in Hydrometallurgy, Vol. 7, pp. 7-26 (1981). Briefly summarized, 50:50 volume percent mixtures of reagents were tested for their ultimate load characteristics under degradative conditions specifically noted to be far removed from commercial plant operating conditions and not providing a suitable basis for commercial choice of one or the other oxime. Some reagent mixtures were reported to exhibit apparent "interactions" providing increased degradation of one or both oxime components (e.g., LIX.RTM.63 mixed with P5100 and SME 529 mixed with P5100) while other mixtures displayed no appreciable differences in degradation properties when compared to unmixed individual components (e.g., LIX.RTM.65 mixed with P5100). Thus, while mixtures of certain oximes at times exhibited diminished hydrolytic stability under the test conditions, none of the mixtures showed statistically significant stability advantages over individual constituents and none of the mixtures were noted to display any favorable kinetic or equilibrium properties in comparison to individual constituents.
Finally, Tumilty, et al., [Adv. in Extractive Metallurgy Int'l. Symp. 3rd, pp. 123-131, 1977 (London)] reported the properties of a reagent series comprising an alkyl substituted hydroxy benzaldoxime extractant and varying quantities of a nonylphenol equilibrium modifier. The report cites experimental studies relating to a proposed plant changeover from a "present generation [LIX.RTM.64N] oxime" to a benzaldoxime reagent having a 3:1 nonylphenol/oxime weight ratio (Acorga P-5300). The studies indicated that over the course of the gradual replacement of LIX.RTM.64N with the "strong" extractant-containing P-5300, there was a gradual and essentially linear progression in stripped solvent copper level and stage efficiencies from the levels and efficiencies of the original reagent until 100 percent replacement with the substitute was attained. These results substantially mirrored those reported in Ritcey, et al., CIM Bulletin, February, 1974, pp. 87-92, wherein mixtures of a "strong", 8-hydroxyquinoline extractant (Kelex 100) and LIX.RTM.64N were noted to be compatible, but not to display either synergism or antagonism.
There continues to exist, therefore, a general need in the art for reagents for solvent extraction proceses for the recovery of copper values which display efficient kinetic and equilibrium characteristics but which include diminished quantities of kinetic additives and/or equilibrium modifiers.